Since shift of the gear stage in an automatic transmission involves change in the rotations of a plurality of rotative elements, the inertia forces of the rotative elements must be absorbed to make the torque shift to be performed smoothly in order to prevent shift shock. Control for preventing the shift shock has been performed by controlling engaging pressure or releasing pressure for frictional engagement units, such as clutches and brakes, for performing the shift so as to absorb the inertia forces (energy) attributable to sliding of the frictional engagement units.
Change in the rotations of the engine which occurs when shift is performed becomes different between a power-on state in which the accelerator pedal has been depressed and a power-off state contrary to the power-on state. Therefore, when clutch-to-clutch shift is performed in which the states of engagement of two frictional engagement units are simultaneously changed, the engaging pressure for the on-coming frictional engagement unit is made adaptable to the state of revolution of the engine when the shift is performed.
When shift down, which is clutch-to-clutch shift, is performed, the engaging pressure for the on-coming frictional engagement unit is gradually raised (swept up) after rise in the engine revolving speed to the synchronized revolving speed at the gear stage set by the shift down because the engine revolving speed is attempted to be raised in the power-on state. In the power-off state, the engine revolving speed is undesirably lowered if the frictional engagement unit which has realized the gear stage is released. Therefore, the engaging pressure for the on-coming frictional engagement unit is enlarged in an early stage to raise the engine revolving speed to the synchronized revolving speed at the gear stage set by the shift down. That is, the engaging pressure for the frictional engagement unit is controlled to be adaptable to the tendency of the change in the revolving speed of the engine when the shift is performed.
When the clutch-to-clutch shift is performed, learning control is performed such that the hydraulic pressure is corrected in accordance with the state of fuel injection in the engine and the tied-up state when the previous shift has been performed and the shift is performed with the corrected hydraulic pressure when the next shift is performed.
That is, the clutch-to-clutch shift is performed such that the hydraulic pressure for at least one of the frictional engagement unit of the frictional engagement units for performing the shift is successively changed to correspond to the state of progress of the shift to prevent shock attributable to rapid change of the output torque. In this case, change in the revolving speed (the engine revolving speed) input to the automatic transmission is affected by the input torque, the friction coefficient of the frictional member or the change rate of the hydraulic pressure. Thus, there arises a possibility that fuel injection in the engine is undesirably performed excessively or a tied-up state occurs on the contrary.
Therefore, the foregoing problems have been prevented by correcting the controlled value of the hydraulic pressure in accordance with the detected state when the shift has been performed and the next clutch-to-clutch shift is controlled in accordance with the corrected controlled value. Since the foregoing control is able to use the individual difference in the automatic transmission and the factor such as the change of the frictional engagement unit as the time lapses in the control of the shift, control of the shift suitable for each case can be performed. Therefore, shift shock occurring when the clutch-to-clutch shift is performed can be prevented more satisfactorily.
Since it is preferable that rapid change in the rotations is prevented in order to prevent shift shock, a throttle valve of the engine has been electronically controlled to also control the engine revolving speed as well as the control the hydraulic pressure for the automatic transmission, in recent years. An example of the foregoing structure has been disclosed in Japanese Patent Laid-Open No. 5-231525 (JPA-5-231525).
The invention disclosed as described above relates to a hydraulic pressure control when so-called synchronizing shift is performed in which the opening of the throttle is enlarged by detecting the shift down when the shift down has been performed in a state where the throttle valve is closed. Thus, the engine revolving speed is synchronized with the revolving speed at the gear stage after the shift and shift down is performed in the foregoing state. Moreover, hydraulic pressure control means for preventing or restraining rise in the line pressure occurring attributable to the temporary enlargement of the opening of the throttle when the synchronizing shift is performed is provided. Thus, shock occurring because of the rapid torque capacity of the frictional engagement unit when the shift down is performed is prevented.
When the above-mentioned clutch-to-clutch shift or direct control of the pressure is performed, initial hydraulic pressure control has been performed in which the hydraulic pressure which is applied to the on-coming frictional engagement unit is temporarily raised simultaneously or immediately after the shifted output has been performed to reduce a so-called pack clearance so as to cause the frictional engagement unit to immediately be provided with a torque capacity when higher hydraulic pressure is applied.
The initial hydraulic pressure control is a control which is capable of bringing the frictional engagement unit into a standby state in which the frictional engagement unit can immediately and substantially be engaged. That is, if insufficient control is performed such that the initial hydraulic pressure is too low, timing for the frictional engagement unit to substantially be engaged is delayed and thus the shift response deteriorates. If the initial hydraulic pressure is too high, the frictional engagement unit is undesirably provided with an excessively large torque capacity. As a result, there arises a risk that a next control of low-pressure standby cannot satisfactorily be performed.
The above-mentioned synchronizing shift is performed at a down shift in a substantial power-off state when a manual selection by a driver is carried out, for example. If the shift down is performed by so-called clutch-to-clutch shift in which engagement/release states of two frictional engagement units are simultaneously changed, the engaging pressure for the off-going frictional engagement unit is relatively early swept up to raise the engine revolving speed to the synchronized revolving speed at the gear stage after the shift. That is, in accordance with the state of the revolution of the engine when the shift down has been judged, the control of the hydraulic pressure for the frictional engagement unit is judged and performed.
On the other hand, the engine is controlled such that the revolving speed is raised in accordance with a fact that the shift down is the synchronizing shift. Since the control to raise the revolving speed is performed by temporarily opening the throttle valve, also the engine torque is simultaneously enlarged.
In this case, the automatic transmission is controlled in accordance with the contents of control in the power-off state and the engaging pressure for the on-coming frictional engagement unit is raised when the shift is completed. However, since the throttle opening is enlarged after the shift has been started, a power-on state is undesirably realized. As a result, the control of the hydraulic pressure for the automatic transmission and the state of the operation of the engine do not coincide with each other. Thus, the revolving speed of the engine is undesirably raised when the shift is completed, thus raising a possibility that the shift shock takes place.
The above-mentioned problem also arises when the foregoing learning control of the hydraulic pressure is performed. That is, the learned value of the hydraulic pressure includes the torque applied to the automatic transmission when the previous shift has been performed. Therefore, if the hydraulic pressure for the synchronizing shift is controlled in accordance with the learned value of the hydraulic pressure when an ordinary shift has been performed, there arises a possibility that the shift shock takes place because the states of the input torque are considerably different from each other.
The foregoing problems also arise when ordinary clutch-to-clutch shift except for the synchronizing shift is performed. If the learned value includes data obtained when the synchronizing shift has been performed, the input torque at a gear shift in which the learned value is obtained differs from the input torque at the gear shift which must be controlled, and the learned value to be used in the gear shift is inadequate. Thus, there arises a possibility that excessive shift shock takes place.
When the foregoing control of the initial hydraulic pressure is performed such that the ordinary shift and the synchronizing shift are controlled in the same manner, the difference between the operation state of the engine when the shift is performed and the input to the automatic transmission may cause an inadequate initial hydraulic pressure at the gear shift. Moreover, there arises a possibility that the following control of the hydraulic pressure during the shift is delayed.
An object of the present invention is to prevent shift shock in so-called synchronizing.
Another object of the present invention is to provide a control unit which is capable of properly controlling learning of the hydraulic pressure in a clutch-to-clutch shift.
Another object of the present invention is to provide a control unit which is capable of adequately controlling the initial hydraulic pressure at a clutch-to-clutch shift or at a shift carried out by directly controlling the hydraulic pressure.